* sly.el: Break long lines

(sldb-frame-menu-map): Use a little macrolet sugar.
Clean up context menus of some sly-part buttons

* sly.el (sly-move-to-source-location): Document NOERROR and
return nil in that case.
(sly--display-source-location): New optional NOERROR param.
(sly-xref-mode): Enter sly-mode and make buffer read-only
(sldb-frame-map): RET bound to `sldb-toggle-details'.
(sldb-frame-menu-map): Simpler menu.
(sldb-local-variable-button, sldb-frame-button): Simpler label.
(sly-inspector-mode-map): Bind D, not d, to
`sly-inspector-describe-inspectee'
(sly-inspector-part): Implement `sly-button-describe'
(sly-inspector-part): Pass `noerror' to
`sly--display-source-location'
(sly-inspector-part-button): Simpler label.
(sly-inspector-describe-inspectee): Add docstring and rename from
`sly-inspector-describe'

* sly-mrepl.el (sly-mrepl--make-result-button): Simpler button label.

* lib/sly-buttons.el (sly-button-popup-part-menu): Fix label.

* lib/lisp/swank.lisp (describe-inspector-part): New slyfun.
(pprint-inspector-part): Fix docstring.
* swank-mrepl.lisp (mrepl-eval): Unbreak IN-PACKAGE